PURIFICATION PLANT FOR REMOVING OF NUTRIENTS FROM SEWAGE

Abstract: The invention concerns a purification plant for removal of nutrients particularly removal of nitrogen from sewage. The new about the invention is that the recirculation is carried out by turbulence in a hydraulic boundary layer in a processtank, where the oxic and anoxic chamber is separated by one or more partition-walls and where the hydraulic boundary layers are placed in one or more openings in the partition walls. The recirculation can be improved by the mixers in the oxic and anoxic chamber.

Description: Description: Name: Purification plant for removing of nutrients from sewage. Field of application: 10 The invention concerns a biological purification system of the ac¬ tivated sludge type for treatment of sewage from households as well as from industries or both. In an activated sludge treatment plant sewage is purified by micro- 15 organisms, hich are stirred in a procestank while supplying oxy¬ gen/air.The microorganisms and the purified water run from the pro¬ cestank to the secondary sedimentation tank,where the microorganisms sedimentates,while the purified sewage runs to the recipient. 20 An activated sludge plant can be designed also to remove the nutri¬ ents:nitrogen and phosphorus.Nitrogen is removed by two biological processes called nitrification and denitrification.At nitrification oxygen is supplied often by atmospharic air, hereby specialised mi¬ croorganisms are oxidizing ammonia into nitrate.Hereafter other mi- 25 croorganisms reduce nitrate into atmospharic nitrogen by using the organic matter in the sewage as reductors with no oxygen present. Phosphorus is removed either biological or chemical or both biologi¬ cal and chemical. 30 The state of the art: t h Removal of nutrients in activated sludge plants have been known for 35 several years. he known technic for removal of nitrogen consists in 4 principal types: SUBSTITUTE SHEET a.recirculation b.alternating c.Treatment plant with only 1 procestank with alternating aerated and unaerated periods. d.Treatment plant with only 1 procestank with aerated and unaerated zones. However the existing methods have a number of drawbacks: a.recirculation:At least 2 procestanks are necessary, hich are cost¬ ly.The degee of recirculation has to be high at high degrees of pur¬ ification,which requires big pumps,a high consumption of energy and causes hydraulic problems. b.Alternating:At least 2 procestanks are required, hich are costly- .Equipment for aeration is required in both procestanks,which also is costly.Movable overflow-weirs or valves in the outlet and inlet are also required since both inlet and outlet is alternating, c.l-tank plant with alternating periods:About one half og the orga¬ nic matter in the sewage is oxidized by the supplied air instead of being oxidized by the nitrate which should be reduced.This results in bad purification and high consumption of energy, d.l-tank plant with aerated and unaerated zones:It is difficult to seperate the zones and it is difficult to operate the recirculation between the zones as well as the size of the zones. The existing treatment systems which are closest to the invention are the recirculation (a) and the socalled OCO-treatment plant.The 0C0-treatment plant is a type "d" treatment plant with a circular procestank.The treatment plant consists of an oxic zone,an anoxic zone and a mixing zone.The oxic zone and the anoxic zone is sepera- ted by a halfcircular partition wall.The mixing zone is the one half of the procestank. The technical problem, hich has to be solved: The technical problem which has to be solved is to avoid the disad- SUBSTITUTE SHEET vantages mentioned above and to minimize the costs of investments and operation. A treatment plant with the following advantages is invented: -the organics in the sewage are used optimal -a high degree of recirculation between the aerated and unaerated zone is obtained -no recirculation pumps are necessary -the recirculation and the size and duration of the zones can be regulated -only 1 procestank is necessary -operation costs are small -investment costs are small. The new technic: These advantages are obtained by designing the procestank (see fig 1) as one tank with an oxic and an anoxic chamber, here the recir¬ culation is carried out by turbulence in a hydraulic boundarylayer in one or more openings in the partition wall between the oxic and anoxic chamber.In the anoxic chamber the reduction of nitrate into free oxygen (denitrification) takes place by using the organics in the sewage as a reductor.The sewage runs either directly into this chamber or indirectly after passing a central tank.The microorgani¬ sms ( the activated sludge) are mixed by one or more mixers,which can be designed as propeller-mixers. hile the anoxic chamber is all- ways unaerated the organic matter in the sewage is used optimal for denitrification. From the anoxic chamber the sewage runs through one or more openings in the partition wall into the aerated (oxic) chamber.In the oxic chamber the sewage is aerated, hereby ammonia is oxidized into nitr- ate.The aerators can either be surface-aerators or submerged aera¬ tors.The activated sludge is mixed by one or more mixers. SUBSTITUTE SHEET From the oxic chamber the nitrate is recirculated by hydraulic dis¬ persion into the anoxic chamber through the openings in the parti¬ tion wall.In the anoxic chamber the nitrate is reduced into free nitrogen, which is transported into the atmospheric nitrogen pool. Each nitrogen-atom passes a lot of times (more than 20 times) thro¬ ugh the openings in the partition wall in and out of the oxic and anoxic chamber.Hereby oxidation of ammonia and reduction of nitrate occurs alternately until all ammonia is almost converted into free nitrogen.In this way a high degree of recirculation is achieved wit¬ hout pumping and thereby a high degree of purification is achie¬ ved.In the traditional recirculation plant pumping is necessary to recirculate.This pumping limits the degree of recirculation because of hydraulic limitations. The transport of ammonia and nitrate between the oxic and anoxic chamber through the two openings take place by turbulent mixing in the boundary layer between the flow in the oxic and anoxic chamber,- whereby the nitrate is transported natural by hydraulic dispersion- from water with a high concentration of nitrate (oxic chamber) into water with a low concentration of nitrate (anoxic chamber) and ammo¬ nia is transported natural by hydraulic dispersion from water with a high concentration of ammonia ( anoxic chamber) into water with a low concentration of ammonia ( oxic chamber). This is astonishing while a designer usually will install pumps to recirculate the water from the oxic chamber back into the anoxic chamber.A designer will be astonished that nitrate moves against the overall flow from the anoxic chamber back into the oxic chamber. A mass-balance across the oxic chamber results in the normal expres¬ sion for the recirculation rate,R, : R = TotN,in /N03,out - 1 = Qr/Q (1) where SUBSTITUTE SHEET TotN,in : total nitrogen in the inlet (g/m3) N03,out : nitrate in the outlet (g/m3) Qr : the recirculation flow (m3/s) Q : the inlet flow (m3/s) If A (m2) is the the area of the openings in the partition wall,the area,A,can be calculated as: A = R*Q/v = ( TotN,in/N03,out - 1 )*Q/v (m2) (2) where v : the velocity of recirculation flow (m/s) The sludge load,SL,is defined as: SL = Q*B0D/V*X (g BOD/g SS day) (3) where BOD : the concentration of BOD in the inlet (g/m3) V : the volume of the procestank (m3) X : the sludge concentration (g/m3) Substituting (3) into (2) you will find the necessary area of the openings as: A = (Totn,ind/N03,out - l)*(SL*X/BOD*v) * V (m2) (4) Inserting different realistic numbers into (4) you will find that approriate areas of the openings will be in the range of 0,05 to o,25 times the area of the partition wall including the openings.If the openings are too large too much oxygen will be transported from the oxic chamber into the anoxic chamber.If the openings are too small the recirculation of nitrate will be too small.From (2) it is seen that the appropriate area of the openings depends on the degree of purification and the wastewater flow. SUBSTITUTE SHEET If the geometry of the procestank is cylindrical the relation bet¬ ween the area of the openings and the area of the partition wall,Ap, including the openings is: A/Ap = ( TotN,in/N03,out -1 )* (SL*X/BOD*v)*0,7*R (5) where R : the radius of the procestank. From (5) it seen that a bigger procestank requires a bigger relative opening in the partition wall. From the oxic chamber the mixture of purified water and activated sludge runs into one or more sedimentation tanks or seperators,where the purified water and the activated sludge is seperate .The activa¬ ted sludge runs back into the procestank whereas the purified water runs to the recipient.If seperators are used,these are placed in the oxic chamber of the procestank. If a central chamber is designed,the central chamber can work as selector,biological phosphorous removal tank,concentration tank or sedimentation tank. The new ideas of the invention is that the recirculation takes place by hydraulic dispersion in a boundary layer in one or more openings in the partition wall between an anoxic and oxic chamber. Thus according to the invention the recirculation takes place by turbulent mass-transfer across a hydraulic boundary layer between two chambers in one tank. Thus the invention is different from other nutrient removal activa¬ ted sludge plants because the plant according to the invention only consists of one tank with an oxic and anoxic chamber with no recir¬ culation pumps.By the dispersion a high degree og recirculation can be obtained which ensures a high degree of purification. SUBSTITUTE SHEET The invention is different from the ordinary recirculation plant because l)it dos only consist of one tank, 2) it has no recircula¬ tion pumps and 3) the degree of recirculation is very high.The in¬ vention is differnet from the OCO-plant because 1) it has no mixing- chamber, 2) the oxic and anoxic chamber is seperated, 3) the deten- tion time of the ammonia and nitrate in each chamber is considerable higher and 4) the recirculation is more proper regulated. The plant has small costs for construction and operation because recirculation pumps are avoided and since the plant dos only con¬ sists of one tank as shown in fig. l.Thus the invention eliminates the drawbacks of the ordinary recirculation plant and the OCO-plant. The effects of the technic. By using the herein described new technic in combination with the known technic the following effects are achieved: 1) A high degree of purification since the degree of recirculation is very high,more than 20 times. 2) Small construction-costs,since only one tank is necessary and recirculation pumps can be avoided. 3) Small costs for energy since the recirculation pumps are avoided. Figurs. Figur 1 shows a plan(l) and a crosssection (10) of a preferred em- bodiment of a purification system according to the invention. In figur 1 a design according to the invention is shown in which there are two openings (2) in the partition wall (5).The partition wall divides the anoxic chamber (3) from the oxic chamber (4).The aeration(7) is placed in one part of the oxic chamber(4) and a se- perator(8) is placed in the other part of the oxic chamber.By pla¬ cing a seperator in the procestank the secondary sedimentation tank SUBSTITUTE SHEET and returnsludge-pumps are avoided.The aeration equipment can be either surface-aerators or submerged aerators.Diffusers with blowers can be used as submerged aerators.Also pure oxygen can be used.One mixer(6) is placed in the oxic chamber(4) and one mixer(6) is placed in the anoxic chamber(3) both mixers placed on the same bridge in front of one of the two hydraulic boundary layers(l) in the openings between the partition walls (5).It is also possible to place a mixer in front of each opening between the two chambers and it is also possible to arrange more than two mixers in the procestank.In the middle of the procestant a central chamber (9) is placed. SUBSTITUTE SHEET

Claims: Claims . l.A purification system of the activated sludge type for removal of organics and nutrients including a procestank.The procestank is de- 5 signed as a tank with a partition-wall dividing the procestank in an anoxic and an oxic chamber,in both chambers one or more mixers are placed and in the oxic chamber aeration equipment is placed ,- characterized in that the recirculation between the oxic and anoxic chamber is carried out by turbulence in a hydraulic boundary layer 10 in one or more openings in the partition wall. 2. purification system according to claim 1,characterized in that the total area of the openenings in the partition wall is between 0,05 and 0,25 times the area of the partition wall including the 15 openings. 3.A purification system according to claim 1,characterized in that a seperator is placed in the oxic chamber. 20 4.A purification system according to claim 1,characterized in that the mixers can be turned and thereby improve the recirculation. 5.A purification system according to to claim 1,characterized in that the mixers have variabel speed, hereby the recirculation can be 25 ajusted. 6.A purification system according to claim 1,characterized in that a central tank is placed in the middle of the procestank and works as a gritcha ber or a selector or a biological phosphorus removal 30 tank or sedimentation tank or a concentration tank or a combination of those. r 7.A purification system according to claim 1,characterized in that more than 2 mixers are placed in the procestank. 35 8.A purification system according to claim 1,characterized in that the aeration is carried out by surface-aerators or submerged aera- SUBSTITUTE tors or pure oxygen. 9.A purification system according to claim 1,characterized in that the aeration can be stopped and started intermittened,whereby the outer chamber is intermittened oxic and anoxic SUBSTITUTE SHEET

Inventors: ANDERSEN KARSTEN KROGH, ANDERSEN, KARSTEN, KROGH
Applicants: ANDERSEN KARSTEN KROGH, ANDERSEN, KARSTEN, KROGH
Application year: 1994
Main classification: C02F3
IPC codes: C02F3